KVM: x86/xen: Remove unneeded xen context from kvm_arch when !CONFIG_KVM_XEN

Saving a few bytes of memory per KVM VM is certainly great but what's more
important is the ability to see where the code accesses Xen emulation
context while CONFIG_KVM_XEN is not enabled. Currently, kvm_cpu_get_extint()
is the only such place and it is harmless: kvm_xen_has_interrupt() always
returns '0' when !CONFIG_KVM_XEN.

No functional change intended.

Reviewed-by: Maxim Levitsky <[e-mail]>
Tested-by: Jeremi Piotrowski <[e-mail]>
Signed-off-by: Vitaly Kuznetsov <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Sean Christopherson <[e-mail]>

arch/x86/include/asm/kvm_host.h

···
 	unsigned long *bitmap;
 };
 
+#ifdef CONFIG_KVM_XEN
 /* Xen emulation context */
 struct kvm_xen {
 	struct mutex xen_lock;
···
 	struct idr evtchn_ports;
 	unsigned long poll_mask[BITS_TO_LONGS(KVM_MAX_VCPUS)];
 };
+#endif
 
 enum kvm_irqchip_mode {
 	KVM_IRQCHIP_NONE,
···
 	struct hlist_head mask_notifier_list;
 
 	struct kvm_hv hyperv;
+
+#ifdef CONFIG_KVM_XEN
 	struct kvm_xen xen;
+#endif
 
 	bool backwards_tsc_observed;
 	bool boot_vcpu_runs_old_kvmclock;

arch/x86/kvm/irq.c

···
 	if (!lapic_in_kernel(v))
 		return v->arch.interrupt.nr;
 
+#ifdef CONFIG_KVM_XEN
 	if (kvm_xen_has_interrupt(v))
 		return v->kvm->arch.xen.upcall_vector;
+#endif
 
 	if (irqchip_split(v->kvm)) {
 		int vector = v->arch.pending_external_vector;